This invention relates to electrical signal recording or writing systems for binary media. Specifically, this invention relates to halftone screening method and apparatus for suppressing false contours in continuous tone images simulated on binary media.
Binary media is intended to refer to media which has resolution elements, picture elements (pels) or pixels that are capable of assuming either of two states, e.g. black or white. Classically, continuous tone images are simulated on binary media by organizing groups of pixels into areas called halftone cells usually m.times.n matrices. The halftone cell has a gray level capability equal to the number of pixels in the halftone cell plus one (for all black or all white). However, in low resolution displays (e.g. CRT and gas panel displays) and graphic recording systems (e.g. xerographic, ink jet and electrostatic), the size of the halftone cell is noticeable to a human observer and can be objectionable. The more pixels included in a cell means that more gray levels can be reproduced. But, as the halftone cell becomes larger, the presence of the cell in the image becomes more objectionable to a human observer.
The process of simulating continuous tone or gray level images with the halftone cell technique has several limitations. One limitation is associated with the ability of the human eye to detect very small changes in density (or intensity) when there are large areas for the eye to compare. False contours are image defects that occur in large areas having comparatively constant densities. An example is a nearly uniform sky in an outdoor photograph. A false contour shows up in the sky in a reproduction because subtle differences in the density of the sky are smaller than the spacing between gray levels capable of being simulaed by the halftone cells of the screen.
Heretofore, false contours are suppressed by increasing the number of pixels within a gray scale by increasing the cell size. The suppression of false contours is of course achieved at the expense of a more noticeable screen pattern in the imaage.